From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics
Date: Tue, 24 Jan 2012 13:40:54 -0500
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Cc: Andreas Dilger <adilger@dilger.ca>,
	Christoph Hellwig <hch@infradead.org>,
	Chris Mason <chris.mason@oracle.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Jan Kara <jack@suse.cz>, Boaz Harrosh <bharrosh@panasas.com>,
	Mike Snitzer <snitzer@redhat.com>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"neilb@suse.de" <neilb@suse.de>,
	"dm-devel@redhat.com" <dm-devel@redhat.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"lsf-pc@lists.linux-foundation.org"
	<lsf-pc@lists.linux-foundation.org>,
	"Darrick J.Wong" <djwong@us.ibm.com>
To: Jeff Moyer <jmoyer@redhat.com>
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On Tue, Jan 24, 2012 at 01:05:50PM -0500, Jeff Moyer wrote:
> - buffered writes and buffered O_SYNC writes, all 1MB block size show 4k
>   I/Os passed down to the I/O scheduler
> - buffered 1MB reads are a little better, typically in the 128k-256k
>   range when they hit the I/O scheduler.
> 
> ext4:
> - buffered writes: 512K I/Os show up at the elevator
> - buffered O_SYNC writes: data is again 512KB, journal writes are 4K
> - buffered 1MB reads get down to the scheduler in 128KB chunks
> 
> xfs:
> - buffered writes: 1MB I/Os show up at the elevator
> - buffered O_SYNC writes: 1MB I/Os
> - buffered 1MB reads: 128KB chunks show up at the I/O scheduler
> 
> So, ext4 is doing better than ext3, but still not perfect.  xfs is
> kicking ass for writes, but reads are still split up.

All three filesystems use the generic mpages code for reads, so they
all get the same (bad) I/O patterns.  Looks like we need to fix this up
ASAP.